1. Field of the Invention
This invention re1ates to an improvement in a valve operation changing system for changing the valve timings of intake or exhaust valve of an internal combustion engine in accordance with engine operating conditions, and more particularly to a hydraulic system for controlling the transfer of a rocker arm from a first position to a second position, and vice versa at a higher speed and at a predetermined suitable timing.
2. Description of the Prior Art
Valve operation changing systems have been applied to various uses in the field of internal combustion engines. For example, the valve operation changing system is used in a so-called dual-mode engine which is so arranged that the valve timing of intake and exhaust valve is changed at a light load engine operating range so as to deactivate some cylinders, thereby carrying out a part-load engine operation.
In general, a gasoline engine of the type wherein charge is previously prepared by mixing air and fuel has a tendency that good fuel economy is obtained at a high engine load operating range. In this regard, in the dual-mode engine, the intake and exhaust valves of some cylinders are kept fully closed to interrupt the supply of air and fuel thereinto thereby to deactivate the cylinders. This relatively increases engine load applied to the remaining cylinders, improving combustion and reducing pumping loss. This effectively improves fuel economy of the engine at the light load engine operating range.
The valve timing changing of the intake and exhaust valves of the dual-mode engine is usually carried out by transferring rocker arms from a first cam for cylinder activation or working onto a second cam for cylinder deactivation or rest in accordance with the engine operating conditions. The first and second cams are formed on a single camshaft and located side by side.
Since the transferring of the rocker arms are usually carried out by the biasing force of springs, it is difficult to obtain a sufficient moving speed of the rocker arms, thereby rendering difficult the valve timing changing during a high engine speed engine operation. Besides, there is a fear that the rocker arms and/or cams are damaged due to the fact that valve lift is initiated by the cam at a timing at which the movement of the rocker arm has not yet been terminated. This impairs the reliability and durability of the conventional valve operation changing system.